1. Field of the Invention
This invention in general relates to communication devices and in particular to receivers for communication devices.
2. Description of the Related Art
Communication receivers (e.g. wireless receivers) are utilized by communication devices for obtaining information contained in signals transmitted by other communication devices. Some receivers include a timing synchronization circuit for generating a timing signal from a received signal. The timing signal provides information regarding the beginning of a data portion of a data packet encoded in a received signal. Some wireless receivers also include a carrier synchronization circuit for detecting a difference between an oscillator frequency of the receiver and an oscillator frequency of a transmitter transmitting the signals (carrier frequency error).
With some receivers, a timing signal is extracted by searching for the peak magnitude of a correlation signal of the received signal with a known pattern. In the WLAN 802.11b communication protocol as set forth by the IEEE, a synchronization portion of a data packet is encoded with a sequence of Barker symbols having a certain polarization pattern. A receiver may use a Barker de-spreader to generate a Barker de-spread signal (a correlated signal) by correlating the received signal with a standard Barker symbol. The Barker de-spread signal ideally should exhibit a sequence of pulses with a certain polarization pattern and spaced apart by the time period of a Barker symbol (Barker symbol interval). The positions of the pulses of the Barker de-spread signal are used to establish a symbol boundary (e.g. Barker symbol boundary). After the symbol boundary is establish, phase errors between pulses are calculated, processed, and averaged to estimate phase error caused by the carrier frequency error. This phase error is used to calculate the frequency error, which is used to correct carrier error for the received signal.
The magnitude of the Barker de-spread signal is a sequence of pulses with same polarization and is periodic with a known period of the Barker symbol interval. Ideally, the de-spread signal should have a signal peak within each symbol interval. In a conventional approach, the magnitude signal of the Barker de-spread signal is truncated into segments, each with a length of a Barker symbol interval. These segments are averaged and a peak of the average is used to generate timing information indicative of a Barker symbol boundary. However, in a noisy, and especially harsh, channel environment, each pulse in the Barker de-spread signal may exhibit multiple peaks due to multi-path interference. Accordingly, the above approach for generating a timing signal in such an environment may be difficult.
Furthermore, with the above approach, timing information is first extracted from a Barker de-spread signal prior to determining carrier frequency error. Such a system reduces the time available for carrier error detection, thus reducing the performance of detecting the timing information and carrier error.
What is needed is improved timing and carrier error synchronization for a communication receiver.
The use of the same reference symbols in different drawings indicates identical items unless otherwise noted.